1. Field of the Invention
The present invention relates to a combustion chamber structure for a spark-ignition engine, and more specifically to a structure for a combustion chamber defined between a bottom surface of a cylinder head and a top surface of a piston in such a manner that the bottom surface of the cylinder head serves as a ceiling wall thereof.
2. Background Art
In recent years, there has been an ever-growing need for improving engine fuel economy from not only economic aspects but also environmental aspects of measures for preventing global warming. In spark-ignition engines, the improvement in combustion efficiency is an effective approach for obtaining enhanced fuel economy, and an increase in compression ratio is one of dominant techniques therefor.
A higher compression ratio can be achieved by reducing a combustion chamber volume relative to a cylinder volume. As a combustion chamber structure suitable for providing a higher compression ratio, a pent-roof type combustion chamber structure, for example, is widely used. This type of combustion chamber structure is characterized in that a ceiling wall of a combustion chamber has an intake-side ceiling wall region and an exhaust-side ceiling wall region each formed in a roof shape, to allow a combustion chamber volume to be reduced while ensuring relatively large intake and exhaust valve diameters. In addition, this type of combustion chamber structure is advantageous in generating an in-cylinder flow, such as a swirl flow (rotational flow about a slinging axis of a piston: horizontal or transverse vortex), a tumble flow (rotational flow in a plane parallel to a sliding axis of a piston: vertical vortex) or a squish flow (flow of air or air/fuel mixture pushed out of a circumferential zone toward a central zone of a piston/cylinder bore during an upward movement of the piston).
For example, Japanese Patent Laid-Open Publication Nos. 08-254126, 08-049546 and 2003-184559 disclose combustion chamber structures for generating various types of in-cylinder flows to provide enhanced combustion efficiency. As seen in illustrated sectional configurations thereof, all of these combustion chamber structures can be considered as the pent-roof type.
FIG. 12 shows a sectional configuration of a conventional typical pent-roof type combustion chamber structure, wherein a piston 93 is at TDC (Top Dead Center). A combustion chamber 94 is a space surrounded by a cylinder bore 12 of a cylinder block 50, a top surface 97 of the piston 93, and a ceiling wall 91 which is a bottom surface of a cylinder head 10 exposed to the combustion chamber 94. The ceiling wall 91 has an intake-side ceiling wall region 91a and an exhaust-side ceiling wall region 91b each formed in a roof shape.
A spark plug 15 is installed in the cylinder head 10 approximately at the radial center of the cylinder bore 12 in such a manner as to allow a sparking end thereof to protrude from the ceiling wall 91 into the combustion chamber 94.
The intake-side ceiling wall region 91a is formed with respective intake openings of two intake ports 21, and provided with two intake valves 19 each adapted to open a corresponding one of the intake ports 21 at a given intake timing. The exhaust-side ceiling wall region 91b is formed with respective exhaust openings of two exhaust ports 22, and provided with two exhaust valves 20 each adapted to open a corresponding one of the exhaust ports 22 at a given exhaust timing. Each of the intake valves 19 and the exhaust valves 20 has a surface exposed to the combustion chamber 94, and a part of the intake-side ceiling wall region 91a and the exhaust-side ceiling wall region 91b is comprised of the exposed surfaces.
While the intake valves 19, the exhaust valves 20, the intake ports 21 and the exhaust ports 22 are actually disposed to be offset from the illustrated sectional positions in a direction perpendicular to the drawing sheet, FIG. 12 shows them in the same sectional plane for purposes of illustration.
The ceiling wall 91 has a circumferential region 91d which is approximately flush with a matching surface of the cylinder head 10 with the cylinder block 50 (more specifically, a matching surface of the cylinder head 10 with a cylinder head gasket (not shown) interposed between the cylinder head 10 and the cylinder block 50). This circumferential region 91d of the ceiling wall 91 is generally called “squish area”.
In reality, even if a higher compression ratio is achieved, for example, by ingeniously designing a combustion chamber structure as described above, such a structure cannot always ensure practically valuable combustion. The reason is that a higher compression ratio is more likely to cause abnormal combustion, such as knocking (these abnormal combustions will hereinafter be referred to generically as “Knocking”), as is well known. From a practical standpoint, this means that the compression ratio can be increased only within a range free of Knocking.
Conversely, it means that, if the occurrence of Knocking can be suppressed, i.e., an anti-Knocking performance can be improved, the compression ratio will be able to be further increased.